Abstract:

The kinetics of cross-polarization (CP) between ¹H and ¹³C nuclei in the crystalline regions of elongated polyethylene fiber was studied. It was found that, with a moderate magic angle spinning (MAS) rate, the CP build-up curves of both orthorhombic and monoclinic crystals of PE can be described by I-I*-S- model. For the fiber sample aligning with the MAS axis in the rotor, the CP build-up curves of both orthorhombic and monoclinic crystals exhibit oscillation, while those of the disoriented samples exhibit monotonic increase. These results demonstrated that, to a certain extent, both orthorhombic and monocline crystals are oriented along the direction of elongation. The degree of orientation of the monoclinic crystal is higher than that of the orthorhombic crystal. Measurement of the kinetics of cross-polarization can be employed as a new technique to determine the degree of orientation of polymer chain.

Key words: solid state NMR, kinetics of cross-polarization, crystalline polyethylene, orientation